Phosphor bronze strip having excellent bending workability

ABSTRACT

A phosphor bronze strip having excellent bending workability and high mechanical strength for use in electronic parts such as terminal connectors wherein;  
     the phosphor bronze strip has a stripe in a cross sectional microstructure revealed by etching;  
     the difference of the maximum and the minimum of tin concentration in the region containing the stripe is between 5 and 40 mass %, preferably 5 to 25 mass % of that of a base material; and  
     the strip is manufactured by the steps of cold rolling to a reduction ratio of at least 45%, final recrystalization annealing to the extent that the mean grain size (mGS) is from 0.5 to 2 μm and the standard deviation of the grain size (σGS) is not more than 1.5 μm, and final cold rolling to a reduction ratio of 20 to 70%.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to phosphor bronze strip with excellent bending workability (formability) for use in electronic parts such as terminal connectors, especially improved in bending workability in press forming.

[0003] 2. Description of the Related Art

[0004] Phosphor bronze strips such as C5210 and C5191 (in conformity with JIS H 3110 and JIS H 3130, respectively. JIS stands for Japanese Industrial Standards) that have outstanding bending workability and mechanical strength are widely employed for such uses as electronic parts like terminal connectors.

[0005] Recent years have witnessed stronger tendencies toward slimmer and smaller electronic parts than ever. From the viewpoint of the above, there have been demanded for more improvement of a phosphor bronze strip having both of high mechanical strength and good bending workability. With regard to a phosphor bronze strip having the same composition, mechanical strength and excellent bending workability are contradictory properties. That is to say, materials having high mechanical strength do not have sufficient ductility and poor bending workability. Conversely, materials having poor mechanical strength have sufficient ductility and good bending workability.

[0006] Meanwhile, in the manufacturing process of electronic parts, the processed parts tend to be smaller than usual although phosphor bronze strip to be used has the same or higher mechanical strength than usual. When processed parts became smaller, the bend radii of bending parts of the parts became smaller. Thus, in bending step, the processed part tends to have wrinkles, orange peel or a crack compared with usual.

[0007] In more severe bending in which the phosphor bronze strip to be used is required to have mechanical strength same as or higher than usual while the bend radii is required to be smaller than usual, defectives which never occurred has come up. These defectives are wrinkles, orange peel or a crack occurred in a processed part of a strip in a low frequency rate. These defectives occurred in a low frequency rate are ones having a deviation of a fraction defective rate has arose within a processing lot or between lots. Conventionally, defectives occurred in the bending part can be obviated by changing a condition of press step or a kind of phosphor bronze strip to be used. However, recently in the supplier, occurred are defectives such that the fraction defective rate extemporaneously arises in press step in which it has been worked with stability superficially. These defectives are revealed in the tendency that phosphor bronze strip to be used is required to have the same or higher mechanical strength than usual while the bend radii became smaller than usual.

[0008] Under such circumstances, the object of the invention is to provide phosphor bronze strip having effectively suppressed defectives revealed by the tendency that phosphor bronze strip to be used has the same or higher mechanical strength than usual while the bend radii became smaller.

SUMMARY OF THE INVENTION

[0009] The grounds on which the various elements that constitute the present invention are restricted will now be explained.

[0010] Regarding to the above defectives, the present invention provides phosphor bronze strip achieving both of high mechanical strength and improved bending workability that can be defined as follows:

[0011] (1) A phosphor bronze strip having excellent bending workability and high mechanical strength characterized in that; the phosphor bronze strip has a stripe in a cross sectional microstructure revealed by etching by an aqueous solution containing hydroperoxide and ammonia;

[0012] the difference of the maximum and the minimum of tin concentration in the region containing the stripe is between 5 and 40 mass %, preferably 5 to 25 mass % of tin concentration of a base material (ΔC_(sn) is 5 to 40 mass %, preferably 5 to 25 mass %); and

[0013] the phosphor bronze strip is manufactured by the steps of cold rolling to a reduction ratio of at least 45%, final recrystalization annealing to the extent that the mean grain size (mGS) is from 0.5 to 2 μm and the standard deviation of the grain size (σGS) is not more than 1.5 μm, and final cold rolling to a reduction ratio of 20 to 70%.

[0014] (2) The Phosphor bronze strip according to the above (1), wherein said phosphor bronze strip is manufactured by a further step of final stress relief annealing to reduce a tensile strength at a rate of from 3 to 10% after the step of the final cold rolling.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is an illustration of stripes observed in cross sectional view of a phosphor bronze strip of the invention. In FIG. 1, “1” shows white stripes. “2” shows a sound region (homogeneous fine-grain microstructure region) in the immediate vicinity of“1”. “3” shows black stripes. “4” shows a sound region in the immediate vicinity of “3”. “5” shows stripes observed in a region of the cross sectional surface. “6”shows a sound region in the immediate vicinity of “5”.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Phosphor bronze of the invention is an copper alloy obtained by addition of tin (Sn) and phosphor (P) to copper (Cu) thereby containing tin at a concentration from 3.5 to 9 mass % conforming JIS. The scope of the invention also includes phosphor bronze containing tin at a low concentration of less than 3.5 mass % and phosphor bronze containing tin at a concentration of at least 9 to 10 mass % conforming standards of CDA (Copper Development Association Inc.). Further, trace addition (e.g. not more than 2 mass % in total) of an element such as Fe, Ni and Zn except for Sn and P to phosphor bronze results in grains comprised of a matrix alloy of copper and tin, use of which alloy in the invention may achieve the same effect.

[0017] When the cross section of commercially available phosphor bronze strip is polished to a mirror-smooth state and etched by an aqueous solution containing hydroperoxide and ammonia, the microstructure is revealed. The grains observed in the microstructure may be isometric, stretched in the rolling direction or compressed in the direction of the thickness according to the temper of the phosphor bronze strip, in other words, to the reduction ratio of final cold rolling.

[0018] Here, assuming that the thickness of the material before the cold rolling is to and the thickness after the cold rolling is t, the reduction ratio X of the cold rolling before the final annealing is defined as

X=(t ₀ −t)/t ₀×100(%).

[0019] When a reduction ratio is more than around 15%, the grain boundaries became indistinct gradually. However, the observed is superficially homogeneous microstructure at any rate.

[0020] However, the wide observation range of the cross section makes it possible to observe a stripe in the homogeneous fine-grain microstructure as shown in FIG. 1 at a specific frequency rate. In the stripe part, it may be difficult to judge a grain size or grains may be coarse. The present inventors inspected in detail the microstructure of defectives occurred extemporaneously in bending step.

[0021] As a result, they have found that there is a correlation between the defectives occurred in bending step and a stripe. Further, they have made extensive and intensive studies about the influence of a stripe on the bending workability and about the factors suppress bending workability. Finally, they have found the phosphor bronze strip of the invention.

[0022] The reason of using an aqueous solution containing hydroperoxide and ammonia in etching a polished mirror-smooth surface of the cross section is that this etchant promotes easy revealing of a stripe in the homogeneous fine-grain microstructure by use thereof, compared with other etchant. The cross sectional surface to be mirror-smooth polished may be selected from the group consisting of the cross section parallel to the rolled surface of the specimen, the cross section perpendicular to the rolled surface of the specimen and parallel to the rolling direction, and the cross section perpendicular to both of the strip and the rolling direction.

[0023] The stripe observed at a specific frequency rate in the homogeneous microstructure is the appearance of unevenness of a microstructure. The unevenness of a microstructure is occurred owing to the difference of the recrystallization behavior i.e. grain growth depending on segregation of tin which is made in melting and casting step and remained after following rolling and annealing. The commercially available phosphor bronze strip having a desired thickness is obtained by the step of only rolling and annealing subsequent to melting material stocks and casting into ingots. In the above conventional manufacturing process, hot forging or hot rolling are not included in general. The phosphor bronze strip manufactured without hot forging or hot rolling contains essentially a stripe with the difference of frequency rate or degree. Therefore, it can be deemed that the stripe is a disordered microstructure occurred inevitably in a manufacturing process of a phosphor bronze strip. In the conventional bending step, stripe has not adversely affected. However, in more severe bending in which the phosphor bronze strip to be used is required to have mechanical strength same as or higher than usual and completed size is required to be smaller than usual, defectives such as wrinkles, orange peel or a crack have come up at an unspecific frequency rate.

[0024] In the present invention, tin concentration of the region containing a stripe is measured, the maximum and the minimum of tin concentration of the region are obtained, further tin concentration of the homogeneous fine-grain microstructure is measured and determined as the tin concentration of a base material.

[0025] In the present invention, EPMA (Electron Probe Micro-Analysis) is used in measuring of tin concentration. EPMA is used because using EPMA makes it possible to analyze trace elements contained in a micro area accurately and also it is an analysis instrument that has become widespread and used for general-purpose.

[0026] In EPMA analysis, an electron beam irradiates a target spot and the intensity of the specific X ray generated from the irradiated spot is measured. The intensity of the specific X ray has a linear correlation with the tin concentration of the spot, thus the tin concentration can be detected. In EPMA analysis, there are many methods such as point analysis, line analysis, surface analysis, mapping analysis. In the invention, line analysis is used preferably. However, the other analyses except line analysis or a combination of a plurality of analyses can be employed in the case that it is ensured that the same or higher measuring accuracy than that of line analysis.

[0027] In line analysis, the region containing a stripe is probed with a moving electron beam. A method in which an electron beam is fixed while a sample is moved, can be employed. Then tin concentration curve is measured. The tin concentration curve is measured on the line from inside of the homogeneous fine-grain microstructure region as a starting point, across the stripe region, to the homogeneous fine-grain microstructure region as an endpoint. When a stripe occupying wide range such as one-fifth to four-fifth of the total thickness of a specimen, or a stripe reaching to the surface of a specimen is observed in the cross section, a tin concentration curve is measured on the line throughout the thickness of a specimen, perpendicular to the surface of the phosphor bronze strip.

[0028] The radius of the electron beam spot irradiated on the measuring line may be the minimum probe size that the analysis instrument to be used is capable. In general, the nominal value of the capable minimum probe size is frequently displayed as 0 μm. The space between the adjacent measuring lines may be 0.01 to 1 μm depending on the width of a stripe. With regard to the other conditions such as an electrical current applied on a specimen, an acceleration voltage, a kind of a spectroscope crystal, a kind of a specific X ray, employed are recommended conditions for tin concentration analysis by the analysis instrument manufacturer. In the above EPMA measurement, the special technique is not necessary and sufficient is the widely used common technique alone.

[0029] In the present invention, the difference of the maximum and the minimum of tin concentration in the region containing a stripe is between 5 and 40 mass % of tin concentration of a base material (ΔC_(sn) is 5 to 40 mass %). The above difference means the degree of tin segregation. With reference to the bending workability, the lower difference is the better. However, reducing the segregation needs thoroughly homogenizing annealing. Especially, it is required to add considerable amounts of heat so as to suppress the segregation from a small degree to zero. The above homogenizing annealing is not favorable since it is necessary to spend enormous cost and extravagant time.

[0030] From the above, in the invention, the inventors have found that the method of improving the bending workability of a conventional material while segregation remains to some degree, in the process conditions described in the latter part of the microstructure of the invention. In the present invention, the highest difference value of the maximum and the minimum of tin concentration is 40 mass %, preferably 25 mass % of tin concentration of a base material. Since, exceeding 40 mass %, further exceeding 25 mass %, even if combination of rolling and recrystalization annealing is optimized, the bending workability more excellent than that of a conventional material cannot be achieved.

[0031] On the contrary, although less than 5 mass % suppresses deterioration of the bending workability owing to segregation and makes it easy to enforce the mechanical strength, it is unfavorable for economic reasons that it is necessary to spend enormous cost and extravagant time for homogenizing annealing to reduce the segregation as described above.

[0032] The stripe region having the difference of the maximum and the minimum of tin concentration more than 40 mass % of tin concentration of a base material, poorly matches with a base material. Further, the deformation function and deformation behavior of the above stripe region are different from these of a base material. Therefore, it is not possible for the above stripe region to follow the deformation of a base material in the plastic deformation in the bending step. Besides, the above stripe region shows different deformation behavior from that of a base material region. As a result, at a boundary between the above stripe region and a base material region, a discontinuous deformation occurs and therefrom wrinkles, orange peel or a crack appears as a starting point. The mechanism of the above is similar to that orange peel or a crack appears by bending step when there is a crack or linked intermetallic compounds of nonmetal(s) inside a base material.

[0033] Further, in the present invention, the phosphor bronze strip is manufactured by the steps of cold rolling to a reduction ratio of at least 45% before final recrystalization annealing. Since, a reduction ratio less than 45% suppresses to obtain fine recrystalized grains having a small mean grain size in final recrystalization annealing. That is, the object of the invention is to hold a balance of the bending workability and the mechanical strength by suppressing the decrease of ductility. The decrease of ductility is suppressed by achieving the desired mechanical strength by means of specified hardening step in which the reduction ratio of final cold rolling is reduced by minimizing the mean grain size before the final cold rolling, i.e., the mean grain size of the final recrystalizing microstructure. The invention can be applied to even the above microstructure having segregation that is a deterioration factor of bending workability, and both of excellent bending workability and the high mechanical strength can be obtained.

[0034] In the steps of cold rolling of the invention, an upper limit to a reduction ratio is not defined. However, excessive high reduction ratio needs too heavy load. Thus, the phosphor bronze strip may be typically manufactured by the steps of cold rolling to a reduction ratio of about 90% or less.

[0035] Here, the reason why the mean grain size (mGS) after recrystarization annealing is from 0.5 to 2.0 μm is that the mean grain size of less than 0.5 μm rather impairs the bending workability after cold rolling with relation to the segregation. Conversely, the mean grain size of more than 2.0 μm inhibit to produce a sufficient effect of hardening step in final cold rolling and it became impossible to obtain a desired mechanical strength at less reduction ratio.

[0036] It is preferred that the recrystalizaiton grain is homogeneous. When the standard deviation of the grain size is more than 1.5μm, even if the mean grain size (mGS) is 2.0 μm or less, the hardening in the final cold rolling step can not be achieved.

[0037] In the following steps, cold rolling to a reduction ratio of 20 to 70% is applied to the phosphor bronze strip. Since, a reduction ratio of less than 20% does not bring a problem of bending workability reduction even if the segregation is retained to some extent. Conversely, exceeding 70%, ductility is impaired and the effect of the minute mean grain size cannot be achieved. Thus, a reduction ratio of 20 to 70% is employed in the following step of cold rolling.

[0038] With regard to phosphor bronze strip, the method of stress relief annealing after the step of final cold rolling is often employed in order to recover ductility and improve bending workability. In the invention, it is beneficial to employ stress relief annealing to reduce a tensile strength at a rate of from 3 to 10% after the step of the final cold rolling even if there is tin segregation and the above process conditions are applied. Here, the conditions such as temperature, atmosphere or tension in stress relief annealing, can be optionally selected and not be restricted in the invention. In stress relief annealing, recrystalization and reduction of tin segregation would not occur by dispersion of atoms.

[0039] The phosphor bronze strip of the invention has an excellent bending workability compared with conventional strip and is expected to show the excellent effect in more severe bending process.

EXAMPLES

[0040] Phosphor bronze stocks of the compositions (in conformity with JIS or CDA) given in Table 1 were charcoal-coated in air, melted, and cast into ingots each having 100 mm wide, 40 mm thick, and 150 mm long.

[0041] The cast ingots were annealed for homogenizing in an atmosphere of 75% N₂+25% H₂ at a temperature from 600 to 800° C. for 0.5 to 3 hours, and the tin segregation layer formed on the surface was removed by means of a grinder. The chemical composition of the ingots was analyzed.

[0042] Cold rolling and recrystallization annealing were then repeated a plurality of times each so that 0.2 mm thick sheets were obtained as products. The microstructure of the product was observed to confirm a stripe. In observation, a stripe may be usually distinguished from other parts because of its color (e.g. white or black) and/or unevenness. Then, the tin concentration curve of the region containing a stripe was measured. Bending workability was determined in bending test.

[0043] (Testing Procedures)

[0044] Confirmation of a stripe contained in the microstructure under an optical microscope was conducted on a cross section perpendicular to the rolling direction. The cross section polished to a mirror-smooth state was etched by an aqueous solution containing hydroperoxide and ammonia, then the microstructure was revealed. As the above aqueous solution, use was made of a mixture of 20 ml of 28% of aqueous ammonia on the market, 10 ml of an aqueous solution obtained by diluting 34.5% hydroperoxide on the market with water to 10 times by volume, and 20 ml of water.

[0045] A tin concentration curve of the region containing a stripe was measured by EPMA line analysis. EPMA was conducted by using JXA8600 manufactured by Japan Electron Optics Laboratory Co., Ltd., Japan. In line analysis, a tin concentration curve was obtained by probing with an electron beam from a direction perpendicular to the rolled surface of the specimen and parallel to the rolling direction.

[0046] The tin concentration curve was measured on the line from inside of the homogeneous fine-grain microstructure region as a starting point, across the stripe region, to the homogeneous fine-grain microstructure region as an endpoint.

[0047] When a stripe was in the wide range such as a quarter to three quarter of the total thickness of a specimen, a tin concentration curve was measured on the line throughout the thickness of a specimen. Here, the probe size of the irradiated electron beam was set at the minimum of the measuring instrument to be used (nominal value:0 μm). The space between the adjacent measured lines was 1 μm.

[0048] Grain size was determined by the intercept method (JIS H 0501) which comprises counting the number of the grains completely sectioned along a predetermined length of segment, and finding the mean value of the cut lengths as the grain size. The standard deviation (σGS) of the grain size was that of thus obtained grain size.

[0049] A scanning electron microscope (SEM) image of the sectional structure perpendicular to the rolling direction was magnified 4,000 times, and each 50 μm-long line segment was divided by the number obtained by subtracting 1 from the number of the points of intersections between the line and grain boundary, to obtain a value as a grain size. The mean of the individual grain sizes determined with 10 segments was deemed as the mean grain size (mGS) and the standard deviation of those grain sizes was deemed as the standard deviation (σGS) of the present invention.

[0050] Bending workability (r/t) was determined in the following way. Each test specimen, 10 mm wide and 100 mm long, was sampled in the transverse direction to the rolling direction and subjected to a W bend test (JIS H 3110) to various bend radii. The minimum bend radius ratio (r (bend radius)/t (thickness of the specimen)) was obtained at which no crack appears. The axis of bending in the W bend test was parallel to the rolling direction (bad way). The fraction defective rate was obtained on the basis of 100 test specimens. In 100 mm length of the specimen, the above test was conducted at a random position where W bend test can be applied. In the evaluation, the specimen showing a crack appeared after the test was evaluated as “not good” and the specimen showing no wrinkles appeared after the test was evaluated as “good”

[0051] In Table 1, each combination of Example 1 and Comparative Example 7, Example 2 and Comparative Example 8, Example 3 and Comparative Example 9, and Example 5 and Comparative Example 10, can be compared since they have the same tin concentration in conformity with JIS or CDA. These Examples shows tensile strength 20 to 50 MPa higher than that of these Comparative Examples.

[0052] In Comparative Example 12 in which mean grain size (mGS) was not 2 μm or less because of cold rolling of low reduction ratio before final recrystalization annealing, in Comparative Example 13 in which mean grain size (mGS) was more than 2 μm after final recrystalization annealing, and in Comparative Example 14 in which there was a scatter of the grain size, the tensile strength was lower than that of Example 3 having the same tin concentration.

[0053] Further, in an example and a comparative example each having the same tin concentration in conformity with JIS or CDA, r/t(s) showing bending workability were the same or r/t of the example was lower than that of comparative example, and the fraction defective rate in bend test was 0%. According to the above, regarding to mechanical strength, it has found that the Examples of the invention showed excellent bending workability while maintaining high mechanical strength.

[0054] Furthermore, in each pairs of Examples 1 and 1′ and Examples 4 and 4′, the different values (ΔC_(sn)) were obtained by means of different conditions of homogenizing annealing. In each pairs, despite of the same reduction ratio of cold rolling, mean grain size (mGS) after final recrystalization annealing, and standard deviation of the grain size, and the same level of tensile strength, r/t were different. As a result, it was found that phosphor bronze strip having 25 mass % or less of ΔC_(sn), shows more excellent bending workability.

[0055] Conversely, in Comparative Examples 7 to 11 in which the values (ΔC_(sn)) obtained by dividing the difference of the maximum and the minimum of tin concentration in the region containing a stripe with tin concentration of the base material was beyond the limitation of the invention, a few percent of fraction defective rate of bending was observed. Further, in Comparative Example 15 in which a reduction ratio of final cold rolling was less than that of the invention, mechanical strength same as that of Examples of the invention could not obtained, although the values (ΔC_(sn)) of Comparative Example 15 was within the limitation of the invention. TABLE 1 After recrys- Reduction ratio of talixation Reduction cold rolling before annealing ratio of Tensile strength (MPa) Fraction Composition recrystalization mGS σGS final cold ΔC_(sn) after cold after stress defective (mass %) annealing (%) (μm) (μm) rolling (%) (%) rolling relief annealing r/t rate Ex. 1 Cu-4.2Sn-0.13P 48 2 1.0 30 30 623 — 1.5 0 1 Cu-4.2Sn-0.13P 48 2.0 1.0 30 20 621 — 1.0 0 2 Cu-6.2Sn-0.13P 50 1.8 1.2 25 28 652 — 1.0 0 3 Cu-8.0Sn-0.13P 50 1.6 1.0 25 18 746 — 1.5 0 4 Cu-8.0Sn-0.13P 50 1.6 1.0 25 18 746 702 0.5 0 4 Cu-8.0Sn-0.13P 50 1.6 1.0 25 30 745 705 1.0 0 5 Cu-10Sn-0.13P 60 1.1 0.7 35 22 901 — 4.0 0 6 Cu-10Sn-0.13P 60 1.1 0.7 35 22 901 819 2.0 0 Com. 7 Cu-4.2Sn-0.13P 40 6.0 2.1 35 45 602 — 2.0 4 Ex. 8 Cu-6.2Sn-0.13P 40 8.2 2.3 30 48 625 — 1.0 2 9 Cu-8.0Sn-0.13P 44 5.0 2.2 25 52 702 — 2.0 3 10 Cu-10Sn-0.13P 40 4.2 2.1 35 44 844 — 3.5 5 11 Cu-8.0Sn-0.13P 50 1.6 1.0 25 48 746 — 2.0 3 12 Cu-8.0Sn-0.13P 44 4.5 1.2 25 18 705 — 1.5 0 13 Cu-8.0Sn-0.13P 50 4.3 1.3 25 17 704 — 2.0 0 14 Cu-8.0Sn-0.13P 50 1.5 2.0 25 17 716 — 2.0 0 15 Cu-8.0Sn-0.13P 50 1.5 1.3 15 17 686 — 2.0 2 

What is claimed is:
 1. A phosphor bronze strip having excellent bending workability and high mechanical strength characterized in that; said phosphor bronze strip has a stripe in a cross sectional microstructure revealed by etching by an aqueous solution containing hydroperoxide and ammonia; the difference of the maximum and the minimum of tin concentration in the region containing the stripe is between 5 and 40 mass % of tin concentration of a base material (ΔC_(sn) is 5 to 40 mass %); and said phosphor bronze strip is manufactured by the steps of cold rolling to a reduction ratio of at least 45%, final recrystalization annealing to the extent that the mean grain size (mGS) is from 0.5 to 2 μm and the standard deviation of the grain size (σGS) is not more than
 1. 5μm, and final cold rolling to a reduction ratio of 20 to 70%.
 2. The phosphor bronze strip according to claim 1, wherein said difference of the maximum and the minimum of tin concentration in the region containing the stripe is between 5 and 25 mass % of tin concentration of a base material (ΔC_(sn) is 5 to 25 mass %).
 3. The phosphor bronze strip according to claim 1 or 2, wherein said phosphor bronze strip is manufactured by a further step of final stress relief annealing to reduce a tensile strength at a rate of from 3 to 10% after the step of the final cold rolling. 